Microwave T-switches are amongst the most common embodiments of coaxial radio frequency (rf) switching devices in communication satellite applications. Microwave T-switches are typically of small size and volume and are well adapted for satellite communication applications that have constrained mass and volume satellite payloads. Conventional rotary coaxial T-switches such as those disclosed in U.S. Pat. Nos. 5,065,125 and 5,063,364 have switch states that are selectable by driving a cam disc to various predetermined angular positions. Actuation means are used to rotate the cam disc within a coaxial microwave switch to the desired angular position and typically utilize either permanent magnet devices or switched reluctance devices.
Permanent magnet devices resemble brushless dc motors and are doubly excited devices in which magnetic flux is generated by a driven coil on the stationary part and a permanent magnet on the moving part. Force is developed through the mutual flux linkages. Generally, permanent magnet devices utilize a relatively large proportion of magnetic material that substantially increases the mass and volume of the actuator. Permanent magnet actuators exhibit residual torque properties, which tend to hold the actuator in preferred locations when un-powered. These effects, which are due to the influences of the magnets, must be overcome when applying power to achieve a new position thereby diminishing the ultimate performance of the actuator. While this un-powered holding torque may be exploited to latch the mechanism between actuations, this is not required in the T-switch application because the load provides sufficient latching torque and the un-powered torque becomes a parasitic effect. The application requirement that the actuator have a well defined, precise target displacement (i.e., a power on equilibrium point where the mechanism comes to rest in the desired location) only serves to exacerbate this parasitic effect.
Switched reluctance devices are singly excited devices with a driven coil on the stationary part and soft ferromagnetic material on the moving part. Force is developed as the moving part tends towards an orientation in which the magnetic circuit reluctance is minimum. Such singly excited actuators have zero un-powered torque. However, because operating torque is related to the change in reluctance with respect to angular displacement, and because there is a finite total change in reluctance possible with available materials and fabrication methods, such actuators only operate efficiently where small angular displacements are required. Since the conventional microwave T-switch requires 60° displacement variable, reluctance actuators are not appropriate for use.